A transforming new form of MSV was released from mouse S plus L-cells. A novel hybrid oncornavirus with dual ecotropic and xenotropic properties was cloned. Several levels of control of xenotropic virus were described. Control of ecotropic virus proteins was detected in synchronized mouse cells. Reversion from MSV transformed state was accomplished by the eliminataion of the MSV genome. Two sets of DNA sequences transcribed from MSV were isolated; that which was MSV specific DNA, and that which was shared with MuLV. MSV specific sequences were found in normal mouse DNA but not in chemically induced tumors. The S plus L-MSV genome was translated in all cell systems in part as a p60 precursor protein. Families of reactive neutralizing antibody were elicited by gp71 and p15(E) proteins in heterologous hosts. Non-antibody factors were found in mouse sera which inactivated only murine xenotropic virus.